In the 2nd Generation (abbreviated as 2G) and 3rd Generation (abbreviated as 3G) mobile communication networks, such as Global System for Mobile communications (abbreviated as GSM) and Universal Mobile Telecommunications System (abbreviated as UMTS), a voice service is provided in circuit switch (abbreviated as CS) domain, and forms a good coverage. With the development of wireless broadband technology and the development of Internet Protocol (abbreviated as IP) technology, the entire network is evolved towards the direction of all-IP. On one hand, the core network of the communication network is evolved towards the IP multimedia core network subsystem (abbreviated as IMS), and on the other hand, the wireless access network is developed towards packet switch; the voice and other services are carried by the packet switch (abbreviated as PS) network, the IMS can be directly accessed, and the IMS provides communication services including the voice service.
The IMS is an IP-based network architecture proposed by the 3rd Generation Partnership Project (abbreviated as 3GPP), which constructs an open and flexible service environment independent of access types, can support multimedia applications, and can provide rich multimedia services to the user.
In the IMS service system, the control layer and service layer are separated, in which the control layer does not provide specific services but only provides necessary functions such as triggering, routing, and charging to the service layer. The service trigger and control function in the control layer is completed by a Call Session Control Function (abbreviated as CSCF). The CSCF is divided into a Proxy (abbreviated as P-CSCF), an Interrogating (abbreviated as I-CSCF), and a Serving (abbreviated as S-CSCF), wherein the Serving is mainly responsible and the Interrogating is optional. The service layer is composed of a series of application servers (abbreviated as AS) for providing particular traffic services. The AS can be an independent entity and can also co-locate with the S-CSCF. The control layer S-CSCF, according to the subscription information of the user, controls service triggering, invokes the services on the AS, and achieves the service functions. The AS and S-CSCF can be collectively referred to as server equipments (SE). The end-to-end equipment in the session is referred to as user equipment (abbreviated as UE) which is responsible for interacting with the user. Some UEs have a plurality of methods to access the network, including accessing the network through the packet switch (abbreviated as PS domain) of 3GPP, accessing the network through the data domain of other non-3GPP, and even accessing the network through the circuit switch (abbreviated as CS) domain, and so on.
For a UE which has a plurality of access methods, whether it can use a plurality of access methods at the same time or can only use one access method at a certain moment, assuming that it is executing a certain service, such as a call service, under an access method, when the UE moves to other place and has to change the used access method, and if the UE and the network can provide a certain method to enable the service being executed by the UE not to be interrupted, such ability is referred to as service continuity, and the application server achieving the service continuity is referred to as a service continuity application server (abbreviated as SCC AS).
FIG. 1 is a service scenario of a voice call transfer (also referred to as voice continuity) under the above scenario. When the user carries out a voice service in a source wireless access coverage area, the user directly accesses the IMS and establishes an IMS voice session with the remote user. When the user moves out of the above coverage area, in order to ensure the continuity of the voice service, the user transfers to the CS network, and the continuity of the call with the remote user is kept by using the session anchoring function in the IMS, so that the good coverage advantage of the CS network can be made the best and the service continuity is ensured. An eMSC (enhanced Mobile Switching Centre) includes functions of a media gateway (divided into a control part MGCF and a media processing part MGW) of the IMS network, which are described by being combined into one entity. In order to simplify illustration and description, the S-CSCF and SCC AS are views as one entity, and they communicate with each other by using a standard Session Initiation Protocol (abbreviated as SIP).
As shown in FIG. 1, before the service continuity procedure occurs, a session is established between the UE-1 and the UE-2, and its signaling paths are described as follows:
A101: is a signaling path between the UE-1 and the P-CSCF which communicate with each other through the SIP protocol of the IMS, and for the SCC AS, it belongs to an access leg path;
A102: is a signaling path between the P-CSCF and the SCC AS/S-CSCF which communicate with each other through the SIP protocol of the IMS, and for the SCC AS, it also belongs to an access leg path;
R101: is a signaling path between the SCC AS/S-CSCF and the UE-2 which communicate with each other through the SIP protocol of the IMS, and for the SCC AS, it is a remote leg path.
After the service continuity occurs, the signaling paths and media paths between the UE-1 and the UE-2 are changed, and the change in the signaling paths is described as follows:
A111: is a signaling path between the UE-1 and the eMSC which communicate with each other through the signaling protocol of the CS domain, and for the SCC AS, it belongs to an access leg path;
A112: is a signaling path between the eMSC and the SCC AS/S-CSCF which communicate with each other through the SIP protocol of the IMS, and for the SCC AS, it also belongs to an access leg path;
R101: is a signaling path between the SCC AS/S-CSCF and the UE-2 which communicate with each other through the SIP protocol of the IMS, and for the SCC AS, it is a remote leg path, and after the service continuity occurs, this remote leg path is not changed.
FIG. 2 is an implementation flow chart of the existing voice service continuity, which describes that the UE-1 directly establishes a voice session with the UE-2 in the IMS domain through the PS access network, and after the UE-1 transfers from the PS domain to the CS domain, the particular description of a process of how the UE-1 and the network implement call service continuity is as follows.
In step 201, the UE-1 initiates a CS call to the eMSC in the CS domain, for example, sends a “SETUP” (call setup) message carrying a session transfer number (STN).
The STN is a transfer number statically configured on the UE, for routing the call setup request in the CS domain to the SCC AS and identifying this session setup request as a session transfer request.
In step 202, the eMSC converts the CS domain call request into an IMS session setup request carrying the media information of the eMSC, and sends to the SCC AS through the CSCF; since all the sessions are anchored at the SCC AS, the SCC AS, according to the information in the session setup request, such as STN, identifies that this session setup request message is the session transfer request initiated by the UE-1, and associates the session which is needed to be transferred and is already established on the source access network by the UE-1.
In step 203, the SCC AS initiates remote leg update, e.g., through a “re-INVITE” message, to update the media information with the remote end UE-2.
In step 204, after the step 203 is finished, the SCC AS replies the session setup message, for example, sends a “200 OK” message carrying the updated media information, and this message reaches the eMSC through the CSCF.
In step 205, the eMSC replies a CS call setup response, such as a “Connect” message, to the UE-1, so as to complete the setup of the access path in the target CS domain.
In step 206, the SCC AS releases the access leg on the source access network.
The above existing method for implementing the session service continuity has the following problem: when the session already set up on the source access network is held by the UE-1 or the UE-2, the eMSC of the target network cannot learn that the session is held, therefore, in step 202, the state information in the media information carried in the eMSC is the default active state; The SCC AS will use the wrong media state to set up a new access path according to the instruction of step 202, and the SCC AS will use the wrong media state information to update the remote leg in step 203. As described above, after the SC transfer flow is completed, it will cause that the held state voice session of the user is connected incorrectly, which leads to incorrect user experience and even affects user privacy.